FIG. 2 shows a prior art semiconductor light-electricity conversion device as shown on pp. 692-701 of the Proceedings of the 16th IEEE Photovoltaic Specialists Conference (1982).
In FIG. 2, the reference numeral 1 designates an n-type silicon substrate (hereinafter referred to as "n-Si substrate"). The numeral 2 designates a p-type silicon layer produced on the n-Si substrate 1. The numeral 3 designates a germanium layer produced on the p-Si layer 2. The numeral 4 designates an n-type gallium arsenide (GaAs) layer produced on the germanium (Ge) layer 3. The numeral 5 designates a p-type gallium arsenide layer produced on the n-GaAs layer 4. The numeral 6 designates a silicon nitride film having a thickness of 600 to 800 .ANG. produced on the p-GaAs layer 5. The numerals 7 and 8 designate electrodes provided in ohmic contact with the p-GaAs layer 5 and the n-Si substrate 1, respectively.
The device will be operated as follows:
When sun light is projected onto the device from above in FIG. 2, current produced by light-electricity conversion of a relatively short wavelength component between the n-GaAs layer 4 and the p-GaAs layer 5 and by conversion of a relatively long wavelength component between the n-Si substrate 1 and the p-Si layer 2 is outputted from the electrodes 7 and 8.
The Ge layer 3 is deposited on the p-Si layer 2 as an amorphous film by a method such as a vapor plating, and it is treated to have a good crystalline structure by a recrystallization technique such as laser annealing. Furthermore, this Ge layer 3 has the function of enhancing the crystalline structure of the n-GaAs layer deposited thereon because the Ge layer 3 has a lattice constant quite close to that of the n-GaAs layer 4.
In the prior art semiconductor light-electricity conversion device of such a construction, as is apparent from the above description, it is impossible to obtain good crystallinity of the n-GaAs layer 4 if it is produced directly on the p-Si layer 2, and therefore, the Ge layer 3 is provided between the p-Si layer 2 and the n-GaAs layer 4. In this case, however, the Ge layer 3 has a narrower band gap energy than that of silicon (germanium and silicon have band gap energies of 0.66 eV and 1.1 eV, respectively), and it absorbs light of the wavelength region which can be converted by the diode constituted by the n-Si substrate 1 and the p-Si layer 2, whereby the light-electricity conversion is reduced.
Another prior art semiconductor light-electricity conversion device is disclosed in an article "GaAs/Ge/Si SOLAR CELLS", by B-Y Tsaur et al, Proc. 17th IEEE Photovoltaic Specialists Conf., pp. 440-444 (1984). In this article it is reported that a light-electricity conversion element comprising GaAs having a pn junction is produced after a germanium layer is coated over the surface of a silicon substrate by electron beam evaporation.
Another prior art device is disclosed in an article "OPTIMAL DESIGN OF HIGH-EFFICIENCY TANDEM CELLS", by John C. C. Fan et al, Proc. 16th IEEE Photovoltaic Specialists Conf., pp. 692-701 (1982). In this article it is reported that a light-electricity conversion element comprising GaAs having a pn junction is produced on a Ge layer deposited on the surface of a silicon substrate by an electron beam vapor deposition process, and the EPD (Etch Pit Density) of that element is investigated.